Hydrodeoxygenation Of Lignin Into Phenols Over Sulfided Mo-based Catalysts

Lignin is one of the most important components of biomass,which is the only renewable feedstock to produce aromatic chemicals in nature.The uninterrupted long-chain structure of lignin is to be broken up in order to get one or more selectively high value-added target products by using the appropriate catalyst,which is an important approach to its comprehensive utilization.Transition metal sulfide catalysts have an important research value of lignin conversion because of its good acidity,sulfur resistance and hydrogenation activity.In order to explore the application of molybdenum sulfide catalyst in lignin conversion reaction,the aromatic model compounds with different C-C bond and C-O bond in the lignin structure was used as probe reactants to study the hydrodeoxygenation reaction of lignin with molybdenum sulfide.In this paper,molybdenum sulfide catalysts with different supports and different metal loadings were firstly synthesized.Moreover,the composition and structure of the catalyst were deeply understood by physical adsorption,XRD,XPS and other characterization methods.The catalytic property of molybdenum sulfide catalyst in eugenol hydrodeoxygenation reaction was studied by using eugenol as a reactant,which is also a lignin monomeric model compound.The results showed that the hydrodeoxygenation activity of MoS₂/AC catalyst with 10 wt%Mo loading for eugenol is the best.Under the reaction conditions of 300 ^oC reaction temperature,3 MPa of initial hydrogen pressure,reacted for 3 hours,the eugenol was completely converted.The yield of the main product 4-n-propylphenol was up to 64.8%,as well as propylcyclohexene,n-propylbenzene,propylcyclohexane as the by-products.Further studies showed that the activity and selectivity of eugenol hydrodeoxygenation reaction were improved obviously over adding Ni or Co into the MoS₂/AC catalyst therfore to form the bimetallic Ni-MoS₂/AC and Co-MoS₂/AC catalysts.Results showed that the addition of Ni mainly promoted the hydrogenation reaction of eugenol,meanwhile,the addition of Co was in favor of hydrogenation and directly deoxygenation reactions simultaneously.The possible pathway of eugenol on MoS₂ catalysts was proposed by intensive studies.Firstly,the C=C double bond between the propenyl chain was isomerized and hydrogenated.The next steps were demethoxylation to produce 4-n-propylphenol,then 4-n-propylphenol was partly dihydroxylation to form the n-propylbenzene,followed by the benzene ring of the n-propylbenzene hydrogenation to produce cyclic olefins or naphthenic hydrocarbons.Finally,the hydrodeoxygenation reaction of different monomers and dimer model compounds were studied in order to deeply understand the activation of the catalyst for different C-C bonds and C-O bonds,and reveals the variation regularity of its activity,selectivity,and stability.Results indicated that conversions of guaiacol,dimeric benzyl phenyl ether and dibenzyl ether were 100%,but the diphenyl ether conversion was only18.9%,which showed that MoS₂/AC catalyst has good activation ability for the?-O-4linkage in lignin structure,but the activation ability for the 4-O-5 bond is weak.This study provides an important scientific basis and theoretical guidance to explore new methods of activating different C-C bonds and C-O bonds in the lignin structure and developing new efficient catalyst system for converting lignin.